The Mouse Formin, FRLα, Slows Actin Filament Barbed End Elongation, Competes with Capping Protein, Accelerates Polymerization from Monomers, and Severs Filaments

Abstract

Formins are a conserved class of proteins expressed in all eukaryotes, with known roles in generating cellular actin-based structures. The mammalian formin, FRLα, is enriched in hematopoietic cells and tissues, but its biochemical properties have not been characterized. We show that a construct composed of the C-terminal half of FRLα (FRLα-C) is a dimer and has multiple effects on muscle actin, including tight binding to actin filament sides, partial inhibition of barbed end elongation, inhibition of barbed end binding by capping protein, acceleration of polymerization from monomers, and actin filament severing. These multiple activities can be explained by a model in which FRLα-C binds filament sides but prefers the topology of sides at the barbed end (end-sides) to those within the filament. This preference allows FRLα-C to nucleate new filaments by side stabilization of dimers, processively advance with the elongating barbed end, block interaction between C-terminal tentacles of capping protein and filament end-sides, and sever filaments by preventing subunit re-association as filaments bend. Another formin, mDia1, does not reduce the barbed end elongation rate but does block capping protein, further supporting an end-side binding model for formins. Profilin partially relieves barbed end elongation inhibition by FRLα-C. When non-muscle actin is used, FRLα-C's effects are largely similar. FRLα-C's ability to sever filaments is the first such activity reported for any formin. Because we find that mDia1-C does not sever efficiently, severing may not be a property of all formins. Formins are a conserved class of proteins expressed in all eukaryotes, with known roles in generating cellular actin-based structures. The mammalian formin, FRLα, is enriched in hematopoietic cells and tissues, but its biochemical properties have not been characterized. We show that a construct composed of the C-terminal half of FRLα (FRLα-C) is a dimer and has multiple effects on muscle actin, including tight binding to actin filament sides, partial inhibition of barbed end elongation, inhibition of barbed end binding by capping protein, acceleration of polymerization from monomers, and actin filament severing. These multiple activities can be explained by a model in which FRLα-C binds filament sides but prefers the topology of sides at the barbed end (end-sides) to those within the filament. This preference allows FRLα-C to nucleate new filaments by side stabilization of dimers, processively advance with the elongating barbed end, block interaction between C-terminal tentacles of capping protein and filament end-sides, and sever filaments by preventing subunit re-association as filaments bend. Another formin, mDia1, does not reduce the barbed end elongation rate but does block capping protein, further supporting an end-side binding model for formins. Profilin partially relieves barbed end elongation inhibition by FRLα-C. When non-muscle actin is used, FRLα-C's effects are largely similar. FRLα-C's ability to sever filaments is the first such activity reported for any formin. Because we find that mDia1-C does not sever efficiently, severing may not be a property of all formins. Non-muscle cells contain a variety of actin filament-based structures, including lamellipodia, ruffles, filopodia, microvilli, and sarcomeric contractile structures (including cytokinetic actin rings and stress fibers). Assembly mechanisms for these structures are being vigorously investigated. Spontaneous nucleation of actin monomers occurs very slowly (1Pollard T.D. Cooper J.A. Annu. Rev. Biochem. 1986; 55: 987-1035Crossref PubMed Google Scholar), and specific actin-associated proteins that promote rapid actin assembly are required for creating each actin-based structure. Arp2/3 1The abbreviations used are: Arp2/3, actin-related protein 2/3 complex; BSA, bovine serum albumin; DTT, dithiothreitol; FH1 and FH2, formin homology domains 1 and 2; FRL, formin-related gene in leukocytes; GST, glutathione S-transferase; mDia1, mammalian Diaphanous formin; NEM, N-ethylmaleimide. complex is a well characterized nucleation factor, forming networks of branched actin filaments that are present in lamellipodia and ruffles (2Higgs H.N. Pollard T.D. Annu. Rev. Biochem. 2001; 70: 649-676Crossref PubMed Scopus (546) Google Scholar). In contrast, the proteins controlling assembly of many other actin-based structures have not been identified. Formins are a conserved class of actin-associated proteins that have been found in all eukaryotes examined and accelerate filament assembly independently of Arp2/3 complex (3Wallar B.J. Alberts A.S. Trends Cell Biol. 2003; 13: 435-446Abstract Full Text Full Text PDF PubMed Scopus (310) Google Scholar). Two unifying structural features of formins are the Formin Homology 1 and 2 (FH1, FH2) domains, generally found in the C-terminal half of the protein. The FH1 domain contains proline-rich sequences capable of binding profilin and SH3 (Src homology 3) domain-containing proteins. The FH2 domain forms a multimeric structure (4Zigmond S.H. Evangelista M. Boone C. Yang C. Dar A.C. Sicheri F. Forkey J. Pring M. Curr. Biol. 2003; 13: 1820-1823Abstract Full Text Full Text PDF PubMed Scopus (218) Google Scholar, 5Moseley J.B. Sagot I. Manning A.L. Xu Y. Eck M.J. Pellman D. Goode B.L. Mol. Biol. Cell. 2004; 15: 896-907Crossref PubMed Scopus (224) Google Scholar). Budding yeast formins Bni1p and Bnr1p are required for the assembly of actin cables and cytokinetic actin rings in vivo (6Evangelista M. Pruyne D. Amberg D.C. Boone C. Bretscher A. Nat. Cell Biol. 2002; 4: 32-41Crossref PubMed Scopus (165) Google Scholar, 7Sagot I. Rodal A.A. Moseley J. Goode B.L. Pellman D. Nat. Cell Biol. 2002; 4: 626-631Crossref PubMed Scopus (395) Google Scholar). Bni1p has barbed end nucleation activity in vitro, for which the FH2 domain is sufficient (7Sagot I. Rodal A.A. Moseley J. Goode B.L. Pellman D. Nat. Cell Biol. 2002; 4: 626-631Crossref PubMed Scopus (395) Google Scholar, 8Pruyne D. Evangelista M. Yang C. Bi E. Zigmond S. Bretscher A. Boone C. Science. 2002; 297: 612-615Crossref PubMed Scopus (578) Google Scholar). Bni1p also slows barbed end elongation, while blocking complete barbed end elongation inhibition by capping protein (4Zigmond S.H. Evangelista M. Boone C. Yang C. Dar A.C. Sicheri F. Forkey J. Pring M. Curr. Biol. 2003; 13: 1820-1823Abstract Full Text Full Text PDF PubMed Scopus (218) Google Scholar, 5Moseley J.B. Sagot I. Manning A.L. Xu Y. Eck M.J. Pellman D. Goode B.L. Mol. Biol. Cell. 2004; 15: 896-907Crossref PubMed Scopus (224) Google Scholar, 9Pring M. Evangelista M. Boone C. Yang C. Zigmond S.H. Biochemistry. 2003; 42: 486-496Crossref PubMed Scopus (190) Google Scholar). Thus, although Bni1p slows elongation, it allows filaments to elongate in the presence of capping protein. Because capping protein usually caps newly assembled filaments within seconds, formins may allow prolonged filament elongation in cells. In fission yeast, Cdc15 is required for actin ring assembly in vivo. Cdc12 is a barbed end nucleator when bound to the actin monomer-binding protein profilin. In the absence of profilin, Cdc12 caps barbed end elongation A.L. Pollard T.D. J. Cell Biol. 2003; PubMed Scopus Google Scholar). contain at formin have been in with being the characterized to In to cytokinetic actin stress and lamellipodia A. Y. S. J. PubMed Scopus Google Scholar). of stress A. S. Nat. Cell Biol. PubMed Scopus Google Scholar, A. A. M. Y. Mol. Biol. Cell. PubMed Scopus Google Scholar). In vitro, a construct of FH2, and C-terminal domains is a actin filament nucleator F. H.N. Curr. Biol. 2003; 13: Full Text Full Text PDF PubMed Scopus Google Scholar), yeast formins. to barbed from capping protein 5Moseley J.B. Sagot I. Manning A.L. Xu Y. Eck M.J. Pellman D. Goode B.L. Mol. Biol. Cell. 2004; 15: 896-907Crossref PubMed Scopus (224) Google and we the biochemical properties of the mammalian formin, gene in first from as the FRLα S. I. Mol. Cell. Biol. PubMed Scopus Google Scholar), although a of other in the We to the C-terminal of FRLα which contains the complete FH1 and FH2 domains, as well as the In a construct of the in its C-terminal S. I. Mol. Cell. Biol. PubMed Scopus Google Scholar), FRLα-C is a dimer and has multiple effects on muscle FRLα-C binds filaments with an In FRLα-C slows barbed end elongation with an of 2 that it binds to filament barbed This inhibition of elongation is and FRLα-C the barbed from complete elongation inhibition by capping protein. In polymerization FRLα-C actin polymerization in a with a being at FRLα-C FRLα-C's polymerization activity is that for F. H.N. Curr. Biol. 2003; 13: Full Text Full Text PDF PubMed Scopus Google Scholar). FRLα-C also actin creating new barbed capable of with actin that FRLα-C has effects on non-muscle We that polymerization acceleration by FRLα-C is to nucleation and filament severing. In we that FRLα-C's multiple effects on actin are to its ability to with filament sides, with a preference for the side of the barbed construct of FRLα and by from from with and with and The for FRLα (FRLα-C) and for with The from and FRLα-C and with construct and to in yeast and with and at to for and the at on in 1 DTT, and 1 and by and with but with to a of the to and the for protein from the and with for which to a of FRLα-C further enriched by protein with and in 2 at at with in protein activity from expressed as F. H.N. Curr. Biol. 2003; 13: Full Text Full Text PDF PubMed Scopus Google Scholar). In to of its nucleation activity at profilin E. with construct from and to in yeast with and at 1 and an at at on in and by a and to The with 1 1 1 2 1 with and 1 with in 1 DTT, and for an 2 in Profilin at protein a from and and muscle from muscle actin from J.A. S. J. Biol. Full Text PDF PubMed Google and with T.D. Cooper J.A. Biochemistry. PubMed Scopus Google Scholar). and actin on S. Pollard T.D. Cell. Full Text PDF PubMed Scopus Google Scholar), which to polymerization actin from actin with at for at in a a with DTT, with a from to and as a at to a of and actin by of and to and and at for at actin at for at in a in DTT, and a and in for actin at for at in a The of and used for in which actin a in also actin monomers with polymerization properties and and Arp2/3 complex a with and the a and an FRLα-C in 1 1 at and and 2 by at partial specific and the and in at with a of of FRLα-C and in the as for at and for and at at used to and used to the used to the at and and in at multiple and to a in an of The of in the Scholar). the multiple and to a with a of The and an for of for 2 at in polymerization DTT, and 1 1 by of This actin to in polymerization in the absence presence of binding proteins. in to a of to at at for at in a of and in of of the with of in of and by also by is in a H.N. Pollard T.D. Biochemistry. PubMed Scopus Google Scholar). and actin in to an actin of the actin This to by at in to by of and to a of with the by proteins for 1 to rapid to actin to the in a The between of and of for each and between and of from polymerization at the of polymerization with to filament the of to barbed with a rate of (1Pollard T.D. Cooper J.A. Annu. Rev. Biochem. 1986; 55: 987-1035Crossref PubMed Google to the is filament in is to and is at is by the is in is of in and and are of and actin in Because FRLα-C slows elongation rate by filament from in the presence of FRLα-C by actin for at 1 at to in the presence of and at for in a The in polymerization to by a of and to at capping protein, of to by and at for In between FRLα-C and capping protein, these proteins to at of 2 monomers and profilin in to the filaments with a by and and the for elongation by the of in the actin barbed and 1 In between capping protein and of formin binding to barbed end by elongation to a binding model as H.N. Pollard T.D. Biochemistry. PubMed Scopus Google Scholar), a of for capping protein binding to barbed for 2 at in polymerization to 1 in polymerization with This a with a of polymerization with FRLα-C capping protein. the 1 of 1 DTT, at to with with to reduce on a 2 and with a for and and all filaments with on each in filaments being for each on filament for at 1 at in polymerization from and at of protein by and for the at and with to with to severing. the polymerization but of FRLα-C and to to we used to reduce and as for and all filaments with for each in filaments being on filament for at 1 at in polymerization of actin with of FRLα-C for at of to 1 with actin monomers and for at and with 2 of to with as for the severing and each filament with and for each from A.L. Pollard T.D. J. Cell Biol. 2003; PubMed Scopus Google Scholar, I. J. Cell PubMed Scopus Google Scholar). for 2 at in polymerization to of to in 1 DTT, with of forming a to in and for 1 with of and with of with of actin filaments a and with to any the first FRLα-C to in with filaments in the in that be as severing sequences for and FRLα-C and the sequences the and of severing for each of actin from actin at 2 on to from the subunit of Arp2/3 profilin from H.N. Pollard T.D. J. Cell Biol. PubMed Scopus Google Scholar), and capping protein subunit by Cooper and from the the of the of and by The of of by and for protein by FRLα-C a expressed a C-terminal construct of FRLα as a glutathione protein in and from GST, FRLα-C with an of by with a of a of and a of of FRLα-C at in that a with an of These that FRLα in a The that dimer is not FRLα-C and first characterized FRLα-C's effects on muscle actin on non-muscle FRLα-C binds to actin actin FRLα-C construct of binds with an of F. H.N. Curr. Biol. 2003; 13: Full Text Full Text PDF PubMed Scopus Google Scholar). with F. H.N. Curr. Biol. 2003; 13: Full Text Full Text PDF PubMed Scopus Google Scholar), filament binding by FRLα-C is by the presence of of binds filaments with to FRLα-C We FRLα-C's on barbed end elongation from actin FRLα-C slows elongation as by of with actin The for is 2 We to yeast formins (4Zigmond S.H. Evangelista M. Boone C. Yang C. Dar A.C. Sicheri F. Forkey J. Pring M. Curr. Biol. 2003; 13: 1820-1823Abstract Full Text Full Text PDF PubMed Scopus (218) Google Scholar, 5Moseley J.B. Sagot I. Manning A.L. Xu Y. Eck M.J. Pellman D. Goode B.L. Mol. Biol. Cell. 2004; 15: 896-907Crossref PubMed Scopus (224) Google Scholar, A.L. Pollard T.D. J. Cell Biol. 2003; PubMed Scopus Google Scholar), FRLα-C may partially block filament barbed in the elongation further we filament in the presence of barbed E. D. J.A. Pollard T.D. J. Mol. Biol. 2001; PubMed Scopus Google Scholar). filaments a to in the absence presence of FRLα-C capping protein and by FRLα-C slows filament and which at This is not capping protein allows a and FRLα-C from FRLα-C's on actin polymerization by polymerization FRLα-C polymerization of actin monomers but with a new filament at FRLα-C We filament at polymerization and an elongation rate of on of and for barbed and (1Pollard T.D. Cooper J.A. Annu. Rev. Biochem. 1986; 55: 987-1035Crossref PubMed Google Scholar), and from the inhibition of barbed end elongation by FRLα-C. FRLα-C of filaments with a of of filaments to FRLα-C dimer in the FRLα-C the polymerization is as the to polymerization a not FRLα-C's ability to accelerate polymerization is on the of actin monomers a from to an in the of filaments assembled the required to polymerization Profilin the of FRLα-C on is a actin monomer-binding protein that actin nucleation and to filament T.D. Annu. Rev. PubMed Scopus Google Scholar). Profilin binds sequences in the FH1 domain of and the FH1 domain can actin monomers for nucleation F. H.N. Curr. Biol. 2003; 13: Full Text Full Text PDF PubMed Scopus Google Scholar). we the of profilin on elongation from actin filaments and polymerization from monomers in the presence of FRLα-C. profilin that binds of the FRLα-C's inhibition of elongation from to occurs with Profilin has on capping elongation inhibition these Profilin FRLα-C's acceleration of polymerization from monomers in a with profilin filament and to polymerization FRLα-C with for the of protein binds filament barbed preventing FRLα-C elongation in the presence of profilin. We FRLα-C barbed capping protein, to and (4Zigmond S.H. Evangelista M. Boone C. Yang C. Dar A.C. Sicheri F. Forkey J. Pring M. Curr. Biol. 2003; 13: 1820-1823Abstract Full Text Full Text PDF PubMed Scopus (218) Google Scholar, 5Moseley J.B. Sagot I. Manning A.L. Xu Y. Eck M.J. Pellman D. Goode B.L. Mol. Biol. Cell. 2004; 15: 896-907Crossref PubMed Scopus (224) Google Scholar). When FRLα-C and capping protein are to FRLα-C elongation rate in a and with an for barbed of Thus, FRLα-C and capping protein for barbed end but with to other has on barbed end elongation This with an construct the FH1 domain and in the presence of profilin to block the nucleation activity of F. H.N. Curr. Biol. 2003; 13: Full Text Full Text PDF PubMed Scopus Google Scholar). barbed end capping by capping protein, with to FRLα-C for barbed FRLα-C a we found that FRLα-C actin When FRLα-C with actin filaments in by stabilization with a in filament and When to filaments to FRLα-C severing and We that to the barbed end binding ability of and barbed end capping proteins of filaments and H.N. Pollard T.D. J. Cell Biol. PubMed Scopus Google Scholar). we to filament a of filament not by the presence of capping protein of filament Because of filaments with FRLα-C occurs in the that filaments be by an of FRLα-C side binding on filament is not an on FRLα-C and and are FRLα-C with filaments for which in a filament of with of filaments 2 of with filaments for in a filament of and of filaments 2 The filaments by with filaments from to barbed end elongation from we a filament to those Pollard T.D. Curr. Biol. Full Text Full Text PDF PubMed Scopus (218) Google Scholar). actin with FRLα-C monomers and and the new monomers to Thus, the filaments with the newly of these filaments with of actin monomers to end and of each filament with of and for and the for filaments with FRLα-C and Thus, FRLα-C filaments slows barbed end severing activity of FRLα-C we actin filaments of to to by of FRLα-C severing be as the of in the filaments when a in the of a filament and when be for of filaments with FRLα-C these not may have been to of filaments of the of of severing with for by FRLα-C the of FRLα-C the in filament be in an elongation rate of When FRLα-C with by with monomers, we a in elongation rate with filaments a in filament FRLα-C barbed end elongation of FRLα-C on FRLα-C's effects on non-muscle actin, we actin and non-muscle from This actin used for all FRLα-C slows barbed end elongation from actin filaments with the we with muscle actin The for is to the with muscle Profilin not reduce FRLα-C inhibition of elongation from actin not in to its on muscle FRLα-C elongation inhibition by capping protein on actin with a to that on muscle We also FRLα-C's ability to accelerate polymerization from actin actin monomers muscle actin monomers to FRLα-C's effects and the in of filaments and in not we FRLα-C's ability to sever actin of FRLα-C with actin filaments in a of filament from to and Thus, FRLα-C on muscle and non-muscle actin, with the being on elongation In we found FRLα-C to be a dimer that has multiple effects on muscle FRLα-C bound to actin filament In FRLα-C bound and partially actin filament barbed as by its inhibition of barbed end elongation, its inhibition of filament and its inhibition of complete barbed end capping by capping protein. not barbed end elongation, capping protein, that further barbed end binding by formins. Profilin partially FRLα-C's inhibition of elongation from actin FRLα-C also polymerization from actin monomers, an that be partially to its ability to sever In we find FRLα-C effects on non-muscle it barbed end elongation complete barbed end capping by capping protein, polymerization from monomers, and This is the first to severing by any formin. FRLα-C is an in with biochemical for a Bni1p FH2 construct J.B. Sagot I. Manning A.L. Xu Y. Eck M.J. Pellman D. Goode B.L. Mol. Biol. Cell. 2004; 15: 896-907Crossref PubMed Scopus (224) Google Scholar). Bni1p FH2 construct (4Zigmond S.H. Evangelista M. Boone C. Yang C. Dar A.C. Sicheri F. Forkey J. Pring M. Curr. Biol. 2003; 13: 1820-1823Abstract Full Text Full Text PDF PubMed Scopus (218) Google Scholar). the FH2 forms a C-terminal sequences for formins. Because construct contains the of FRLα does not to with that FRLα-C may be Because of a dimer is to the of further FRLα-C is in binding to the filament side barbed end may the The ability of formins to barbed end has been and a of inhibition has Cdc12 barbed end elongation A.L. Pollard T.D. J. Cell Biol. 2003; PubMed Scopus Google Scholar), FRLα-C elongation from muscle actin filaments Bni1p (4Zigmond S.H. Evangelista M. Boone C. Yang C. Dar A.C. Sicheri F. Forkey J. Pring M. Curr. Biol. 2003; 13: 1820-1823Abstract Full Text Full Text PDF PubMed Scopus (218) Google Scholar), and inhibition allow elongation at a formins may the of the filament by side binding the barbed end in a that to We the of formin are sufficient for elongation In F. H.N. Curr. Biol. 2003; 13: Full Text Full Text PDF PubMed Scopus Google Scholar), (4Zigmond S.H. Evangelista M. Boone C. Yang C. Dar A.C. Sicheri F. Forkey J. Pring M. Curr. Biol. 2003; 13: 1820-1823Abstract Full Text Full Text PDF PubMed Scopus (218) Google Scholar), and FRLα not barbed end barbed end elongation, formins with the barbed for capping by Bni1p have been by Zigmond (4Zigmond S.H. Evangelista M. Boone C. Yang C. Dar A.C. Sicheri F. Forkey J. Pring M. Curr. Biol. 2003; 13: 1820-1823Abstract Full Text Full Text PDF PubMed Scopus (218) Google and J.B. Sagot I. Manning A.L. Xu Y. Eck M.J. Pellman D. Goode B.L. Mol. Biol. Cell. 2004; 15: 896-907Crossref PubMed Scopus (224) Google Scholar), which that Bni1p with the barbed end as it This is to be on at formin subunit bound while the other to a new barbed end actin capping for it binds to actin filament that FRLα-C elongation of 2 that FRLα-C to the barbed FRLα-C bound with to the side and barbed end, those be required to We a for FRLα-C binding to elongating barbed FRLα-C binds to actin filament sides and barbed but barbed end binding is FRLα-C does not on the end but binds the side of the barbed end binding partially by monomers, elongation formins the barbed end to in the of elongation inhibition FRLα When a does the interaction between FRLα-C subunit and the actin subunit the the FRLα-C subunit to and the new barbed end The other FRLα-C subunit bound to its actin with the actin filament The other formin subunit model those of J.B. Sagot I. Manning A.L. Xu Y. Eck M.J. Pellman D. Goode B.L. Mol. Biol. Cell. 2004; 15: 896-907Crossref PubMed Scopus (224) Google and Zigmond (4Zigmond S.H. Evangelista M. Boone C. Yang C. Dar A.C. Sicheri F. Forkey J. Pring M. Curr. Biol. 2003; 13: 1820-1823Abstract Full Text Full Text PDF PubMed Scopus (218) Google in that we formin binding to the sides of barbed end of on the barbed end The for formin binding preference at the the side is We with Zigmond (4Zigmond S.H. Evangelista M. Boone C. Yang C. Dar A.C. Sicheri F. Forkey J. Pring M. Curr. Biol. 2003; 13: 1820-1823Abstract Full Text Full Text PDF PubMed Scopus (218) Google that binding is not by actin subunit FRLα-C not and F. H.N. Curr. Biol. 2003; 13: Full Text Full Text PDF PubMed Scopus Google barbed end from We that the end-side preference from topology between and monomers FRLα-C's on elongation from muscle actin elongation inhibition by This that profilin binds to of to barbed be by FRLα-C actin profilin not has been to with profilin in and in and interaction occurs its FH1 domain S. I. Mol. Cell. Biol. PubMed Scopus Google Scholar). Profilin the elongation rate by a that partial barbed end by FRLα-C. FRLα-C filament barbed from complete elongation inhibition by capping protein. This inhibition of capping protein has a as elongation supporting model that FRLα-C at the barbed end as the filament is and is not binding and with each new FRLα-C to be at blocking capping protein at elongation FRLα-C and The structure of capping protein A. Y. J. 2003; PubMed Scopus Google has to of a model in which C-terminal each at the barbed end in an A. Y. Cooper J.A. Curr. Biol. 2003; 13: Full Text Full Text PDF PubMed Scopus Google Scholar). This side end-side binding by formin end-side The that does not elongation but capping protein at to those of FRLα-C is and the end-side binding by Zigmond (4Zigmond S.H. Evangelista M. Boone C. Yang C. Dar A.C. Sicheri F. Forkey J. Pring M. Curr. Biol. 2003; 13: 1820-1823Abstract Full Text Full Text PDF PubMed Scopus (218) Google and by J.B. Sagot I. Manning A.L. Xu Y. Eck M.J. Pellman D. Goode B.L. Mol. Biol. Cell. 2004; 15: 896-907Crossref PubMed Scopus (224) Google Scholar), the ability to capping protein a for the elongation of in FRLα-C slows elongation, in vivo filaments capping protein elongate efficiently, while filaments 1 by capping protein. This a for filaments found in and as to the branched filaments in S. J. Cell Biol. 2003; PubMed Scopus Google Scholar). FRLα-C a property not for formins. This property may be to does not is that the severing is to the side binding and barbed end binding In the absence of barbed FRLα-C binds to the filament side The filament to its creating for FRLα-C to as it does at the filament barbed end When the filament FRLα-C of the barbed end, preventing re-association and severing. FRLα-C it with the new barbed end as it In to mDia1-C does not sever efficiently, which may be to properties that are to side binding and of barbed end is a to FRLα-C's acceleration of polymerization from monomers we in for the The with of that FRLα-C for filaments to nucleate filament FRLα-C's on polymerization is on actin by FRLα-C with of profilin. may not be the to polymerization acceleration in vitro, FRLα-C also nucleation to other formins. The partial inhibition of of FRLα-C may also accelerate polymerization in by the filament We that nucleation is also to end-side which are to by actin dimers, but by binding the dimer end and not the Arp2/3 complex may a dimer elongation the barbed end (2Higgs H.N. Pollard T.D. Annu. Rev. Biochem. 2001; 70: 649-676Crossref PubMed Scopus (546) Google Scholar). protein binds in a barbed end elongation the end J.A. Pollard T.D. Biochemistry. PubMed Scopus Google Scholar). Formins be a variety of from the side and elongation in the effects of FRLα-C on non-muscle actin are to those for muscle actin, are FRLα-C elongation from actin filaments from muscle actin We a with the inhibition on muscle in to muscle profilin does not any to for these be to in the structure of the barbed of non-muscle which may of the barbed end by FRLα-C. for these be the presence of proteins in of the actin reported by the we in the actin to In Arp2/3 complex present in In we not profilin, capping protein, to and Arp2/3 complex from the actin not may and the of FRLα-C. is which biochemical properties of FRLα-C are in cells. The severing and polymerization acceleration activities are that filaments to of actin-based cellular structures. other binding proteins may severing nucleation by FRLα-C. protein inhibition is a and is a for creating filaments in cells. We for of and and for of protein We also of and for with of Goode and Zigmond of for and and Pollard for profilin capping protein, and